1. Field of the Invention
The present invention relates to a developing device, a process cartridge, and an electrophotographic image-forming apparatus.
2. Description of the Related Art
In a typical electrophotographic image-forming apparatus, a developer supply method is used in which developer is added as it is consumed. In a typical developer supply method, developer is supplied by replacing a developer container which is detachably attached to the electrophotographic image-forming apparatus with another one. For example, Japanese Patent Laid-Open No. 8-30084 discloses a developer container including a developer circulation path for making developer contained in the developer container and newly supplied developer uniform before they reach a developing member. In addition, instead of the structure in which two screws are arranged adjacent to each other so that the developer is circulated horizontally, a structure in which two screws are vertically arranged so that the developer is circulated vertically may also be applied, as disclosed in Japanese Patent Publication No. 4-39073 and Japanese Patent Laid-Open No. 10-142942.
However, in the above-described developing device having a structure for circulation, the following points must be considered.
That is, developer in the developing device becomes gradually degraded as it is circulated multiple times in the developing device. This is because resin contained in the developer becomes abraded or deformed by being mechanically rubbed in the developing device. In addition, external additive applied to the surface becomes separated or embedded into the resin. Thus, developer contained in the developing device becomes gradually degraded. Therefore, characteristics, such as charging characteristics, of the degraded developer and those of new developer are different from each other. If the degraded developer and newly supplied developer are conveyed to the developing member while they are unevenly distributed, uneven density or fog appears on an image due to differences in the amount of charge between the new and old developers. Accordingly, the new and old developers must be mixed uniformly at least before they are fed to the developing member.